A sour hydrocarbon fraction is one that contains offensive sulfur compounds such as mercaptans and hydrogen sulfide. These hydrocarbon fractions are treated using a process commonly known as sweetening. Sweetening processes involve reacting the mercaptans in the sour hydrocarbon fraction with an oxidizing agent in the presence of an oxidation catalyst and an alkaline agent to oxidize the mercaptans to disulfide. The oxidizing agent is most often air. When the concentration of mercaptan sulfur in the hydrocarbon fraction is about 5 ppm or less, the hydrocarbon fraction is said to be sweet. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fractions as well as naphtha, kerosene, jet fuel, fuel oil, and the like.
Another method of eliminating mercaptans contained in a sour hydrocarbon fraction is by use of hydrodesulfurization which is also well known in the art. However, hydrodesulfurization involves the use of large quantities of hydrogen which is both uneconomical and hydrogenates some of the desirable components contained in the hydrocarbon fraction. For these reasons hydrodesulfurization is not used to remove mercaptans from a sour hydrocarbon fraction.
Although mercaptan oxidation will usually sweeten a sour hydrocarbon fraction, there are occasions when adequate sweetening is not possible. The apparent reason for this is that the sour hydrocarbon fraction contains a high concentration of tertiary mercaptans which are extremely difficult to oxidize. By tertiary mercaptans is meant mercaptans in which the carbon attached to the mercaptan sulfur atom is also attached to three other carbons. If the concentration of mercaptans is still relatively high after the sweetening process, the value of the product will be lowered. Therefore, there is a need for a process which can economically remove the tertiary mercaptans contained in a sour hydrocarbon fraction.
Applicants have solved this problem by combining a mercaptan hydrogenolysis step with a mercaptan oxidation step. The hydrogenolysis step is a selective hydrogenolysis step which hydrogenolyses the tertiary mercaptans. The conditions used to selectively hydrogenolyse the hydrocarbon fraction are very mild compared to conventional hydrotreating conditions. For example, applicants' process uses only about 0.1 to about 100 cubic feet of hydrogen per barrel of hydrocarbon fraction versus 1,000 to 5,000 cubic feet per barrel required for hydrotreating. Further, the instant process is run with the hydrogen and hydrocarbon fraction in a single phase, i.e., liquid phase, whereas hydrotreating involves a liquid and a gaseous phase. Finally, the selective hydrogenolysis process does not alter the major components of the hydrocarbon fraction.
The other step in the process is an oxidation step where the mercaptans are oxidized to disulfides by contacting the hydrocarbon fraction with an oxidation catalyst. The hydrogenolysis step and oxidation step can be carried out in any order. That is, the hydrogenolysis step can be carried out before or after the oxidation step.
Although the prior art discloses hydrotreating and selective hydrogenolysis, there is no mention of a hydrogenolysis step in combination with an oxidation step to sweeten sour hydrocarbon fractions containing tertiary mercaptans. One reference dealing with selective hydrogenation is U.S. Pat. No. 4,897,175. The '175 patent discloses a selective hydrogenation process for removing color bodies and color body precursors from a hydrocarbon fraction. However, there is no hint nor suggestion in the '175 patent that this process could be used to hydrogenolyse tertiary mercaptans in a sour hydrocarbon fraction. Nor is there any suggestion that a selective hydrogenolysis process could be combined with a mercaptan oxidation step to sweeten a sour hydrocarbon fraction. It is applicants who have recognized the synergistic relationship of a selective hydrogenolysis step followed by an oxidation step to sweeten a sour hydrocarbon fraction.